During rotary drilling, the torque applied to the drill head can wind up the drill string several turns, distributed over the drill string length. This is in turns that the drill head lags the rotary table input at the surface. If the drill head is suddenly lifted off bottom, the torque at the drill head approaches zero and the drill head races forward as the string rotationally unwinds. The lower end of the unwinding drill string can overshoot the neutral position and cause torque in the opposite direction. The reversal from the overrun position tends to unscrew the bottom hole assembly from the drill string. The tendency to unscrew connections can take place anywhere along the drill string. That sequence of events can be prevented under most normal circumstances by an experienced driller, by avoiding sudden removal of bit load while the string is delivering torque to the drill head. Sudden release of bit load, due to unexpected circumstances, sometimes happens. The lower end assembly unscrewed from the drill string and loose down hole is a classic drilling problem that is expensive to correct.